Streptococcus gordonii is a Gram-positive oral commensal bacterium. An initial colonizer of the tooth surface, S. gordonii uses an array of adhesins to adhere and promote biofilm formation. Without these adhesins the bacterium faces extinction from the oral cavity. Upon gaining access to the bloodstream the bacterium can disseminate and adhere to host tissues resulting in opportunistic infections, namely infective endocarditis. S. gordonii readily attaches to and form biofilms on abiotic and biotic surfaces using the numerous adhesins it possesses. A family of these adhesins, termed LPXTG-motif adhesins, is important for mono- and multi-species biofilm formation on surfaces. When the processing of these adhesins is blocked or individual LPXTG-adhesins are deleted, S. gordonii senses the absence of the LPXTG-adhesins and upregulates alternative adhesins to preserve adhesion ability and biofilm formation. The upregulation of alternative adhesins is dependent on an uncharacterized, membrane-embedded, two-component signal transduction system (TCS), SGO_1180-1181. Using genetic studies, the absence of the C-terminal peptide (C-pep) of processed LPXTG-adhesins was identified as a signal for the loss of the respective adhesin. It is hypothesized that the C-peps and sensor kinase of the TCS interact in the cell membrane in a ligand/receptor partnership to sense the proper placement of LPXTG adhesins on the cell surface and regulate alternative adhesins to maintain the attachment and biofilm forming capacity of the cell. The application aims to identify and characterize the molecular interaction between C-peps and SGO_1180 and the resultant signaling cascade responsible for upregulation of alternative adhesins. Complementary in vitro and in vivo studies will characterize and determine the molecular interactions between C-peps and sensor histidine kinase, SGO_1180. Studies will investigate how the presence of C-peps influences the phospho-signaling of SGO_1180 and identify the key amino acids that mediate this interaction. Parallel studies will be conducted to identify the global regulatory impact of interactions between C-peps and SGO_1180 and identify directly regulated genes of the TCS. Ultimately, the goal is to understand the mechanism by which S. gordonii regulates adhesins and prevents eradication from the oral cavity and extinction.